Seed Production of Vegetable, Tuber and Spice Crops

Botany

• Lettuce is one of the most important vegetable crops in temperate countries.Lettuce is an annual crop, when the vegetative growth reaches a mature stage, stem elongation occurs and reproductive development begins. Stems vary in length and thickness. A single stem is usually formed but additional stem may be formed from axillary buds. Caulin or stem leaves are usually narrow and closing at the back.
  
• The inflorescence is a panicle, composed of many capitula or flower heads, indlucing a terminal head. Each capitulum consists of several florets, usually from 12 to 20, but as few as seven and as many as 35. The florets are all ray type, perfect and fertile, and are surrounded by three to four rows of brackets, forming an involucre. Each floret consists yellow, liqulate petal with five teeth. The lower part is fused as a tube and surrounds the sexual parts.
  
• Each floret has a double carpet, consisting of an elongated style and a divided stigma. There are five stamens, the anthers are fused to form a tube. The flowers open only once, in the morning, remaining open for about 1 hr on a warm sunny morning and for several hours when it is cool and cloudy. As the flower opens, the style elongation while the anthers dehisce from within and the shed pollen is swept upwards by the style and stigma hairs.
  
• The ovary is below the corolla, when fertilized it forms and embryo surrounded by nuclear and endosperm tissue and a thin pericarp. The whole is called an achene and is a form of fruit rather than a seed. The achene is topped by a hair like pappus. Achene matures about 2 weeks after fertilization. They may be black, grey, white, brown or yellow.



Figure 1. Inflorescence of lettuce